1. Field of the Invention
The present invention relates generally to hearing prostheses, and more particularly, to hearing prostheses that provide electrical stimulation to the auditory nerve.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive or sensorineural. In many people who are profoundly deaf, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due absent, damaged or destroyed hairs in the cochlea that normally transduce acoustic signals into nerve impulses. Various hearing prostheses have been developed to provide individuals suffering from sensorineural hearing loss with the ability to perceive sound.
One type of hearing prosthesis, referred to as a cochlear implant, includes an electrode assembly implanted in the cochlea. Electrical stimulation signals are delivered to the auditory nerve via the electrode assembly, thereby inducing a hearing sensation in the implant recipient.
Conductive hearing loss occurs when the normal mechanical or acoustical pathways which conduct sound to the cochlea are impeded. This problem may arise, for example, as a result of damage to the ossicular chain or ear canal. Individuals suffering from conductive hearing loss frequently retain some form of residual hearing because the hairs in the cochlea are undamaged. For this reason, individuals who suffer from conductive hearing loss typically are not candidates for a conventional cochlear implant system because insertion of the electrode assembly into the cochlea may severely damage or destroy the remaining hairs in the cochlea.
Individuals with conductive hearing loss typically receive an acoustic stimulation hearing aid. Such aids can also benefit individuals with sensorineural hearing loss who have sufficient residual hearing to not be candidates for a cochlear implant. Hearing aids receive ambient sound, amplify the sound, and direct the amplified sound through the ear canal. The amplified sound reaches the cochlea and causes motion of the cochlea fluid, thereby stimulating the hairs in the cochlea.
Unfortunately, hearing aids do not benefit all individuals with conductive hearing loss. For example, some individuals are prone to chronic inflammation or infection of the ear canal. Other individuals have malformed or absent auricle(s) and/or ear canal(s) as a result of a birth defect, or as a result of a medical condition such as Treacher Collins syndrome or Microtia.
Individuals unable to benefit from hearing aids may benefit from implantable hearing prostheses that deliver mechanical energy to the recipient. In one type of implantable hearing prosthesis, referred to a “middle ear mechanical stimulation system,” an implanted actuator is connected to the ossicular chain, thereby enabling direct vibration of the ossicular chain to induce an auditory response. In another type of hearing prosthesis, referred to as an “inner ear mechanical stimulation system,” an implanted actuator is connected to the cochlea and operates by directly vibrating the cochlea thus causing vibrations in the perilymph.
Another type of acoustic stimulation hearing prosthesis, referred to as a bone conduction device, such as a Baha®, has an actuator implanted into the skull of the recipient. The actuator provides vibrations directly to the recipient's skull bone. These vibrations are conducted by the recipient's bony structure to the inner ear to elicit an auditory response.
Some individuals are provided a bilateral hearing system to treat sensorineural hearing loss. A bilateral hearing system refers to a system in which the individual is provided with a hearing prosthesis for both ears. For example, some individuals with bilateral sensorineural hearing loss are provided a bilateral hearing system comprising a cochlear implant for one ear and a hearing aid for the other.